1. Field of the Invention
The present invention relates to a multilayer balanced filter including a stack including a plurality of dielectric layers and electrode layers.
2. Description of the Related Art
A typical conventional band pass filter whose size and cost can be easily reduced is formed by providing a plurality of LC resonance circuits within a stack including dielectric layers and electrode layers.
Such a multilayer band pass filter is disclosed in Japanese Unexamined Patent Application Publication No. 2005-45447.
The configuration of a multilayer band pass filter disclosed in Japanese Unexamined Patent Application Publication No. 2005-45447 is described with reference to FIG. 1.
FIG. 1 is a circuit diagram of the multilayer band pass filter disclosed in Japanese Unexamined Patent Application Publication No. 2005-45447. The multilayer band pass filter 1 includes an unbalanced input terminal 2, balanced output terminals 3A and 3B, and a band pass filter portion 4 provided between the unbalanced input terminal 2 and the balanced output terminals 3A and 3B. The band pass filter portion 4 is constituted by three resonators 40 arranged in a line, where each oscillator is a ½-wavelength resonator 40 whose both ends are open. The resonator 40 arranged at a position closest to the unbalanced input terminal 2 among the three resonators 40 is an input resonator 40I. The input resonator 40I is directly connected to the unbalanced input terminal 2. The resonator 40 arranged at a position closest to the balanced output terminals 3A and 3B is a balanced output ½-wavelength resonator 41A. The balanced output ½-wavelength resonator 41A is connected to the balanced output terminals 3A and 3B. The input resonator 40I is electromagnetically coupled with an intermediate resonator 40M, and the intermediate resonator 40M is also electromagnetically coupled with the balanced output ½-wavelength resonator 41A. The open ends of the three resonators 40 are connected to the ground by capacitors C.
In the multilayer band pass filter disclosed in Japanese Unexamined Patent Application Publication No. 2005-45447, a balanced-unbalanced conversion function and filter characteristics are realized by arranging a plurality of ½-wavelength resonators having open ends on the same surface of a substrate and thereby coupling the resonators with one another.
In the case of a filter having a balanced-unbalanced conversion function, the balanced terminals thereof are generally often connected to an IC which performs balanced input/output of signals. In this case, impedance conversion is required between the unbalanced terminal and the balanced terminals. However, the structure described in Japanese Unexamined Patent Application Publication No. 2005-45447 does not allow for easy design and adjustment for appropriate impedance conversion between these two types of terminals.
Further, even when a circuit for impedance conversion is provided, an additional area corresponding to the impedance conversion circuit portion is required, thereby preventing general reduction in size.